Arrested development – a comparative analysis of multilayer corona textures in metamorphic rocks

Coronas, including symplectites, are vital clues to the presence of arrested reaction and preservation of partial equilibrium in metamorphic and igneous rocks. Compositional zonation across such coronas is common, indicating the persistence of chemical potential gradients and incomplete equilibration. Major controls on corona mineralogy include P, T and aH2O during formation, continuous or non-continuous corona formation, reactant bulk compositions and extent of metasomatic exchange with the surrounding rock, relative diffusion rates for major components, and/or contemporaneous deformation and strain. High-variance local equilibria in a corona and disequilibrium across the corona as a whole preclude the application of conventional thermobarometry when determining P-T conditions of corona formation, and zonation in phase composition across a corona should not be interpreted as a record of discrete P-T conditions during successive layer growth along the P-T path. Rather, the local equilibria between mineral pairs in corona layers more likely reflect compositional partitioning of the corona domain during steady-state growth at constant P and T. Corona formation in pelitic and mafic bulk rock compositions requires dry, restitic bulk rock compositions. Since most melt is lost at or near peak conditions only a fraction of melt is retained in the restitic post-peak assemblage. Reduced melt volumes with cooling limit length-scales of diffusion to the extent that diffusion-controlled corona growth occurs. On the prograde path, the low melt (or melt-absent) volumes required for kinetically-constrained corona growth are only commonly realised in mafic rocks, owing to their intrinsic anhydrous bulk composition, and in dry, restitic pelitic compositions that have lost melt in an earlier metamorphic event. Mafic and pelitic prograde coronas show similar ranges of thickness and vermicule size; prograde contact aureole coronas display similar thicknesses but slightly longer vermicule lengths compared to regional metamorphic coronas. Retrograde coronas in mafic rocks are significantly thinner than pelitic coronas and have smaller vermicule lengths, whereas retrograde pelitic coronas show similar parameters to their prograde counterparts. Reduced maximum corona thickness and smaller maximum vermicule size in retrograde mafic coronas compared to retrograde pelitic coronas attests to more restricted length-scales of diffusion in melt-poor, anhydrous, mafic bulk rock compositions. Increased maximum layer thickness and vermicule size in prograde mafic coronas compared to retrograde mafic coronas is due to greater length-scales of diffusion in more melt-rich bulk compositions with protracted reaction along the prograde path. Prograde pelitic coronas do not differ significantly from retrograde pelitic coronas with respect to microstructure, owing to the intrinsically more hydrous pelitic bulk compositions and capacity to generate diffusion-enhancing melt during decompression. Through the application of either quantitative physical diffusion modelling of coronas or phase equilibria modelling utilising calculated chemical potential gradients, it is possible to model the evolution of a corona through P-T-X space by continuous or non-continuous processes. Since corona modelling employing calculated chemical potential gradients assumes nothing about the sequence in which the layer forms and is directly constrained by phase compositional variation within a layer, it allows far more nuanced and robust understanding of corona evolution and its implications for the path of a rock in P-T-X space. Key words: corona, chemical potential gradient, diffusion, disequilibrium, metamorphism, mineral compositional zoning, reaction dynamics, reaction texture, symplectite.